In order to supply power from a power supply system (power supply grid) to electric devices, which require a defined supply voltage, power supplies, in particular switching power supplies, are used. These transform a nominal mains voltage (also referred to as mains voltage or nominal voltage in the following), which is supplied by the power supply grid and is contacted with the power supply as an input voltage, to a supply voltage suitable for the electric device. Since the power supply systems installed worldwide differ from each other and can comprise nominal voltages of different magnitudes depending on the area of application, a plurality of power supplies is required for covering the large spectrum of nominal voltages.
According to the state of the art, two groups of power supplies relating to the input voltage range of the power supply have established themselves on the market—AC/DC power supplies for nominal voltages of 24 V to 60 V (also called U1-NT) and AC/DC power supplies for nominal voltages from 100 V to 240 V (also called U2-NT).
Different producers distribute these power supplies on the market as individual devices or power supply modules.
These known power supplies have an input protection circuit which is calibrated to the respective input voltage range, with the dynamic range of the input voltage being 4:1 at most.
In order to be as effective as possible, efforts are made to keep the power loss of the input protection circuit as low as possible.
In a U1-NT, the bridge rectifier is therefore equipped with Schottky diodes which have a maximum reverse voltage of 200 V. The threshold voltage is only 0.3 V in this instance since an input current of 0.75 A and an output load of 10 W flow in small input voltages of 19.2 V (“24 V−20%”) when the efficiency is 70%. The power loss across the 2-fold diode path is 0.5 W in this instance.
In other impedances, such as current-compensated chokes (German: Drosseln or Drosselspulen), efforts are made to construct said impedances with an as small as possible ohmic resistance value in order to not cause any further additional losses.
The energy storage means arranged on the output side in the input protection circuit is dimensioned in the form of a buffer capacitor such that its ESR (Equivalent Series Resistance) only generates small loses in high circuit currents.
In a U2-NT, however, other priorities apply for the input protection circuit. For this purpose, the components must be designed for larger voltages since a voltage of up to 400 V is contacted with the DC intermediary circuit.
The currents flowing through the longitudinal elements are significantly smaller in this range, i.e. only 0.14 A flow in an input voltage of 100 V and an efficiency of 70% as well as in output performances of 10 W.
Schottky diodes as rectifier elements are rejected due to a too small maximum reverse voltage of approx. 200 V. Therefore, silicon bridge rectifiers having a reverse voltage of 1 kV are preferably used in this instance.
The energy storage means (buffer capacitor) takes up a large construction space in the U2-NT. The ESR of the U2-NT is not a crucial factor due to the relatively small circuit currents flowing therein but the type of electrolyte capacitor used in a U2-NT cannot be used in the same manner for a U1-NT because of the large heat build-up.
In some product norms, an impulse voltage test of 4 kV symmetric and 2 kV asymmetric is required. In practice, however, the most switching power supplies are constructed only for lower requirements of 2 kV symmetric and 1 kV asymmetric.
The relatively large construction space required for power supplies and power supply modules available on the market has proven to be a further disadvantage, since it complicates the integration into other devices.
It can be therefore said that thus far two power supply varieties had to be produced for the market, whose respective input protection circuit either covers the lower (U1-NT) or the upper (U2-NT) mains voltage range. Accordingly, the number of devices distributed by manufactures is doubled while simultaneously increasing costs in development, manufacture, storage and dispatch, for example.